An automobile is mounted with many items of electrical equipment, such as audio equipment, an air conditioner damper, an electrically-driven retractable mirror, and a steering lock pin. To drive these items of electrical equipment, a large number of small electric DC motors are used, and an appropriate small electric DC motor is used depending on the torque necessary for driving each item of electrical equipment. The output torque of the small electric DC motor is proportional linearly to the input current. For the automobile, since the voltage applied to the motor is supplied from a battery having a fixed discharge voltage (13.5 V), a higher current must be carried to the motor for electrical equipment requiring a higher torque.
Therefore, the small electric DC motor for automotive electrical equipment is used appropriately according to its initial starting current (hereinafter, abbreviated to IS (Initial Start)). That is, the small electric DC motor used in such a manner that the IS is lower than 1 ampere is used for electrical equipment requiring a low torque, such as audio equipment and an air conditioner damper, and the small electric DC motor used for high-capacity applications in which the IS is 1 ampere or higher is used for electrical equipment requiring a high torque, such as an electrically-driven retractable mirror and a steering lock. For these motors, the kind and configuration of a sliding contact material used for a brush and a commutator are different depending on the current capacity.
The configuration of the small electric DC motor used for high-capacity applications in which the IS is 1 ampere or higher is generally such that a block-shaped carbon-based sintered material one side of which is several millimeters is used as the brush, a Cu-based material is used as the commutator, and the block-shaped sintered material is pressed against the commutator with a spring. In the high-capacity applications, since the carrying current is high, the brush side is greatly worn by a relatively high spark discharge produced when the brush separates from the commutator, or by an arc discharge. To solve this problem, by using carbon that has a low coefficient of dynamic friction against a metal and is less worn mechanically at the sliding time, the durability is assured, and further, by using a block-shaped brush material having a large volume, the wear is compensated.
The above-described motor for high-capacity applications in which the IS is 1 ampere or higher has great durability against current load; however, it has a problem that the rotation noise thereof is loud, and noise is generated frequently. The major cause for this is carbon, which is the brush material. Carbon is a material in which the minimum arc current is low, and is active in producing electric discharge, so that a spark discharge and an arc discharge are liable to be produced at the sliding time, and resultantly rotation noise and noise are liable to be generated. Also, one cause for this is the Cu-based material forming the commutator. The Cu-based material is liable to be corroded by environmental factors. Therefore, a film of oxides, sulfides, or the like is formed easily, so that the contact resistance tends to become unstable, which results in the generation of noise.
In the recent automobile industry, a tendency toward high grade has further increased, and therefore a motor of such quality that will generate less extra noise on a small part such as an electrically-driven retractable mirror and a steering lock tends to have the preferance. Also, on the recent automobile, the control systems for various items of electrical equipment, safety circuits, and the like have become complicated. In view of preventing the malfunction of these control systems as well, noise generation is unfavorable.
In the case where the reduction in rotational noise and noise is considered, as a countermeasures therefor, it has been contemplated to adopt a configuration similar to the small electric DC motor used for applications in which the IS is lower than 1 ampere is adopted. In this low-capacity motor, a sliding contact material of precious metal alloy is mainly used as the brush and the commutator, and is assembled to the motor as a mode of clad material in which the precious metal alloy is embedded in a base material consisting of Cu and Cu alloy. Also, the brush has a structure such that the above-described clad material fabricated into a plate shape or a rod shape is brought into contact with the commutator by utilizing the spring properties of the base material.
The motor using the precious metal alloy for the brush and the commutator as described above can be anticipated in suppressing noise because the precious metal has high discharge resistance, and therefore corrosion is less liable to be produced by environmental factors. Also, in addition to the viewpoint of the material characteristics, by using, on the brush side, the clad material consisting of the base material having spring properties, the follow-up properties of the brush following up the commutator are enhanced. Therefore, noise is also reduced from the structural viewpoint.
However, the motor using the precious metal alloy as described above can be used only for low-capacity applications (in which the IS is lower than 1 ampere). If such a motor is used for high-capacity applications (in which the IS is 1 ampere or higher), the motor stops before the required service life is reached. The mechanism for the stop of motor is thought as described below. For the precious metal alloy having high discharge resistance as well, electric discharge is somewhat produced, that is, spark discharge and arc discharge are produced at the moment when the brush separates from the commutator. This electric discharge melts and wears the slit part of the commutator (the end part of the commutator). Thereby, transfer is produced from the commutator to the non-sliding part of the brush, the transferred substance is again transferred to the slit part of the commutator, and is further transferred to the sliding part of the commutator, so that, finally, mechanical wear occurs between the transferred substances on the commutator. As a result, the brush is broken, and the motor stops before the required service life is reached.
From the above-described consideration concerning the mechanism for motor stop, it can be said that in order to enable the motor using the precious metal alloy to be used for high-capacity applications in which the IS is 1 ampere or higher, the sliding contact material forming the commutator must be restrained from melting and wearing. As for the configuration of the sliding contact material for the motor used for low-capacity applications, there have been known an AgPd alloy used for the brush, and an AgCu alloy described in Patent Literature 1 and an AgCuNiZnOMgO-based alloy obtained by improving the AgCu alloy, which is described in Patent Literature 2, the both being used for the commutator. The sliding contact material described in Patent Literature 2 is a material in which MgO and ZnO particles are dispersed in an AgCu alloy matrix so that these metal oxides achieve the lubricating effect in the sliding part to lower the frictional resistance and to improve the wear resistance. However, even if the sliding contact material whose characteristics are improved as described above is used, the material exhibits high wear resistance as compared with the AgCu alloy in high-capacity applications, but the motor will stop due to breakage of brush before the required service life is reached.